The development and application of the miniaturization technology is the major trend of modern science, and the self-assembly technology, in particular, is the rudimentary method of the microscopic world in the recent years.
Speaking of the micro rotary fan manufactured by Microelectromechanical systems (MEMS) technology, as shown in Appendix 1, the portion between the Scratch Drive Actuator (SDA) of the micro rotary fan and the micro blades structure must be implemented by virtue of the self-assembly technology and multi-user MEMS processes (MUMPs).
The so-called self-assembly technology means that the microstructure will self-align after the completion of the final release process. As shown in Appendix 2, the conventional microstructure fabricated by using of self-assembly technology has the following three types.
Type 1 uses the residual stress from the manufacturing process to generate the deformation resulting in the displacement of microstructure as shown in FIG. 1 of Appendix 2, which illustrates a 3D micro-optic switch developed by Lucent Technology.
Type 2 uses surface acoustic wave generated by ultrasonic wave to move the microstructure to a preset position by vibration as shown in FIG. 2 of Appendix 2.
Type 3 uses the solder ball, photoresist or other polymer to form an elastic joint on the micro-hinge. A molten state of the elastic joint will be present under high temperature reflow process so as to generate a surface tension force pulling up the microstructure as shown in FIG. 3 of Appendix 2.
However, type 1 and type 2 of the traditional self-assembly technology are only applicable to the static application or the fixed microstructure, but not suitable for dynamic or rotated microstructure such as the micro-fan application.
In regard to type 3 self-assembly technology, there are a host of materials suitable for elastic joint fabrication. Different materials featured respective advantages. Take the solder ball as an example:
Lead contamination: The solder ball is composed of tin and lead (63Sn/37Pb). During the reflow process, facilities and environment will be contaminated by lead.
High cost: Most of the surface micromachined microstructures are usually constructed by polycrystalline silicon (Poly-Si), where a layer of gold pad must be coated as an interconnection between the solder ball and Poly-Si. This additional process will inevitably result in production difficulty and cost increasing.
Poor precision: To calculate the raised angle or displacement of microstructure, the dimension of solder ball must be accurately controlled. However, traditional solder ball usually has a volume deviation up to 25%, which makes the precision of the raised angle or displacement uncontrollable.
Manual processing: So far, attaching the solder ball on the gold pad still adopts the manual alignment processing.
Miniaturizing infeasibility: Currently, the smallest diameter of solder ball is no less than 100 μm, which limits the minimum size of the solder-based devices.
Taking the elastic joint formed by photoresist as another example:
The manufacturing process of the elastic joint formed by photoresist is not as complicated as that of the solder ball, and the cost thereof is also lower. However, the release of the microstructure must be processed by dry or wet etching.
The dry etching utilizes liquid carbon dioxide to release the microstructure and replace the water molecule so as to avoid the stick effect of the microstructure. Whereas, the super critical CO2 dry release equipment used for the method is quite expensive, and thus the cost of this process is relatively high.
The wet etching requires no additional manufacturing equipment, making it a solution with less cost. However, after etching the sacrificial layer with the solution of diluted hydrofluoric acid (HF) or buffered oxide etch (BOE), further apply the isopropyl alcohol (IPA) to quickly vaporize the water molecules. The IPA is characterized by dissolving the photoresist so that it will damage the photoresist-based elastic joint fabricated originally.
In sum, considering production cost, process integration and miniaturization capability, a brand new manufacturing process is urgently required to resolve various shortcomings arising from the elastic joint formed by the solder ball or the photoresist.